The present invention relates to a method for producing a tube of glass, particularly of quartz glass, by forming a hollow cylinder from the glass with an outer diameter D1 in that the cylinder, while rotating about a rotation axis, is softened in portions in a heating zone which is moved at a relative feed rate Va, and the softened portion is radially expanded under the action of a centrifugal force and/or of an internal overpressure applied in the hollow cylinder bore so as to form a deformation zone, and the tube is continuously shaped with an outer diameter D2 which is greater than D1 
With such methods and apparatuses, hollow cylinders of glass, particularly of quartz glass, are formed in one or plural hot forming steps into tubes, the radial tube dimensions being changed with respect to the radial dimensions of the hollow cylinder or the cross-sectional profile. An initial hollow cylinder which is rotating about its longitudinal axis is here softened zone by zone and is expanded in this process—under the action of a radially outwardly directed force—either against a molding tool, which is arranged at a predetermined radial distance relative to the longitudinal axis of the tube, or it is formed without tools. The radially outwardly directed force is based on the centrifugal force and/or on an internal overpressure in the inner bore of the hollow cylinder (also called “blow pressure”).
Special attention is here paid to the dimensional stability of the drawn-off tube strand. To ensure this stability, constant detection and continuous control of a radial dimension of the tube strand, such as the outer diameter, the inner diameter or the wall thickness, are indispensable. The blow pressure, the relative feed rate between hollow cylinder and heating zone, and the temperature in the heating zone are common as a control variable of such a control.
Dimensional deviations that already exist in the original hollow cylinder tend to propagate into the drawn-off glass tube during the forming process and are even intensified in this process. Variations in the radial cross-sectional profile or wall one-sidedness; i.e. radially irregular profiles of the tube wall thickness, also called “siding” among the experts, are here particularly disadvantageously noticed. Since upon use of a molding tool, the outer diameter is a relatively fixed given dimension, tube wall one-sidedness is here accompanied by variations in the inner diameter of the tube.
These problems increase with an increasing end diameter of the tube, for wall thickness variations found in the start cylinder exponentially grow with the diameter in the forming process. Therefore, in the final analysis, the maximum values for siding that can still be tolerated according to the specification (e.g., 1 mm) limit the virtually achievable end diameter of the tube. This effect also depends on the level of the blow pressure, so that this pressure cannot be arbitrarily high. Instead of this, in order to achieve commercially acceptable forming rates, the glass must be heated to a higher degree and softened more strongly. This, in turn, leads to more drawing streaks or other defects in the glass wall and to an increased energy demand, especially in the case of large-volume tubes (also called “large tubes” hereinafter) which cool down very rapidly because of their large volume.
The greater the end diameter of the tube, the more difficult and more cost-intensive is therefore the production of a dimensionally stable large tube. To mitigate this problem, it is suggested in JP 2004-149325 A that the forming process should be subdivided into a plurality of forming stages with successive increase in the diameter. For this purpose, the hollow cylinder of quartz glass to be formed, which has a diameter of 250 mm, is clamped in a lathe and is rotated about its horizontally-oriented longitudinal axis while it is heated by means of a ring-shaped arrangement of heating burners and is softened zone by zone in that the heating burners are moved at a predetermined feed rate Va along the cylinder surface. The increase in diameter is due to the centrifugal force acting on the softened portion. The deformation zone will migrate along the whole start cylinder once until the cylinder is fully expanded. The outer diameter of the tube is here continuously captured by means of a laser beam without tools. This forming step will be repeated until the nominal tube diameter of 440 mm is reached. In each forming step, the tube diameter is increased by 15 mm.
CN 102887626 A describes a multi-stage forming process for producing a quartz glass tube with an outer diameter of more than 520 mm with forming stages of 60 mm each.
In this forming process, one achieves a comparatively small forming degree in each individual forming stage, which is accompanied by a smaller deviation from the nominal value of a radial tube dimension. Moreover, each forming stage offers the possibility of considering and correcting dimensional deviations found in the respective start cylinder.
On the other hand, it is evident that this procedure requires a lot of time and energy, especially since the tube cools down between successive forming steps.
The attempt can be made to keep the number of the forming steps as small as possible in that the respective deformation degree, i.e. the change in diameter, is set as high as possible. It has, however, been found that the forming process in the case of very great diameter changes becomes unstable, which first manifests itself in diameter variations that form a wave structure extending in the longitudinal axis direction.